Reactor vessels employed in the nuclear industry, as well as similar vessel used with large industrial facilities, in general, are fabricated as welded, curved plate structures. Typically, reactor vessels will be formed with longitudinal and circumferential seam welds, as well as nozzle welds and the like at their cylindrical or main body portions and with corresponding welds at their hemispherical top and bottom heads. Because of the criticality of maintaining the structural integrity of power reactor vessels over their somewhat extended lifespans, regulatory agencies such as the Nuclear Regulatory Commission (NRC) require extensive examination of the welds and adjacent heat affected zones within predetermined intervals. Typically, non-destructive, in-service examination and evaluation of the welded structures are carried out during scheduled shut-downs planned for such activities as refueling and the like.
Because such planned shut-downs involve a power production outage, the efficiency of their execution is most important to industries. However, the weld inspection procedure is complex, requiring control over worker radiation exposure, and thus calling for remotely controlled examination systems which themselves must be capable of operating within the environment of gamma radiation. Where boiling-water reactors (BWR) or pressurized-water reactors (PWR) are the subject of inspection, advantages have been recognized for an internal approach wherein the water media within the reactor vessel or, additionally, that within the refueling cavity, serve to isolate personnel from radiation originating from the nuclear fuel. Remotely controlled manipulators generally are employed to physically move and position inspection heads or search units carrying ultrasonic inspection transducers and/or eddy-current probes or transducers and the like to positions adjacent to the various vessel weldments and surfaces. Ultrasonic test (UT) and/or eddy-current based examinations are carried out under the control of remote stations, which may be located as far as several hundred feet from the search units mounted on the manipulator. In locating weld flaws, piezoelectric-based transducers or eddy-current probes are excited or appropriately energized by a remotely-derived signal delivered from a control system. The same or another such transducer then reacts for ultrasonic testing to a received echo or an eddy-current response is received to form an evaluating signal that is transmitted for data acquisition to the remote control station.
To achieve continuously reliable examination data during the inspection, it is important that the inspection heads carrying the transducer be properly oriented. In this regard, the transducer should retain a consistent or pre-planned orientation with respect to the curved surfaces of the inner wall of the vessel under inspection. These surfaces of interest may be planar, cylindrical, conical, spherical, parabolic or hyperbolic in nature, including, for example, nozzles. Each such geometry results in a specific pattern of response with respect to the transducer being employed and the general type of surface being inspected is typically known in advance and may be cataloged in computer memory so that digital treatment of received data can be optimized. For ultrasonic (UT) inspection procedures, pulse-echo and "pitch-catch" transducer configurations are employed in the nuclear power field. In the case of the pulse-echo configuration, the transducer, preferably, is oriented along a local normal to the small, local surface under immediate evaluation, or stated otherwise, its forward axis is oriented perpendicularly to the local tangent of the curved surface. For ultrasonic testing of the pulse-echo variety, this orientation assures an appropriate angle of incidence for an inspecting pulse and subsequently refractively affected return or echo signal. Orientation of the inspection head plane also is important with respect to pitch-catch transducer assemblies wherein two transducers are oriented for transmission and reception. Where eddy-current probes are employed, proper "altitude" or "spacing" orientation with a level surface under inspection is important. Due to the remote nature of the examination so carried out, achieving proper orientation and spacing of the transducers and their inspection heads has posed difficlties to practitioners. Typically, the manipulator controlled remote inspection heads will incorporate mechanical "feelers" or fingers which are moved into contact with the vessel interior surface to provide somewhat tactilely based orientation information. Additionally, submersible video imaging systems are employed with the manipulators to observe the interior wall and head positioning.
Present inspection head orientation approaches, however, are limited due in part to the non-uniform nature of the interior surfaces of the vessels. Generally, these walls will be covered with a stainless steel cladding having a rough outer surface. The cladding typically is formed by welding a helix of stainless steel wire to the steel wall of the vessel during its construction. Thus, surface irregularities in the form of cavities, valleys and the like are commonly encountered to disorient the inspection plane of inspection heads employing tactile positioning systems.